Garbage Collector

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BigAdmin Submitted Article: Understanding Concurrent Mark Sweep Garbage Collector Logs. Performance Tuning Garbage Collection in Java. This document is a summary or outline of Sun's document: Tuning Garbage collection with the 1.4.2 Hotspot JVM located here: 1.0 Introduction For many applications garbage collection performance is not significant Default collector should be first choice2.0 Generations Most straightforward GC will just iterate over every object in the heap and determine if any other objects reference it.

This gets really slow as the number of objects in the heap increase GC's therefor make assumptions about how your application runs. Most common assumption is that an object is most likely to die shortly after it was created: called infant mortality This assumes that an object that has been around for a while, will likely stay around for a while. GC organizes objects into generations (young, tenured, and perm) This is important!

2.1 Performance Considerations Ways to measure GC Performance Throughput - % of time not spent in GC over a long period of time.
Garbage Collection: ConcMarkSweep vs. RMI. Abstract Use of the ConcMarkSweep garbage collector for the “tenured” generation is undesirable when RMI (Remote Method Invocation) garbage collection is occurring.

Introduction Java doesn”t require that you explicitly release memory allocations in your program, which greatly reduces the time to build useful applications and eliminates most memory leaks. However, a significant part of any application is spent doing memory management, and in the case of Java, all application threads must be suspended for some period of time during any “garbage collection”, or “GC”. Garbage collection is the operation performed automatically by the JVM that recycles memory that had been allocated for an object, but now is not referenced by any part of the application, and therefore can be reclaimed and reused.

This behavior is not part of the VM specification, however, and is subject to change in future releases. Moreover the behavior described here is generic behavior and will not apply to the execution of all Java applications. How is the generational collector implemented in HotSpot(tm)? The default collector in HotSpot has two generations: the young generation and the tenured generation. Most allocations are done in the young generation. The young generation collector is a copying collector. The tenured generation is collected with a mark-sweep-compact collection.

What is the relevance of -XX:MaxNewSize? The young generation is set by a policy that bounds the size from below by NewSize and bounds it from above by MaxNewSize. Are all eden-space objects moved into the survivor space so that after a minor gc, eden-space is empty? Yes. No. TargetSurvivorRatio does not usually make a big difference.
Tuning Garbage Collection with the 5.0 Java[tm] Virtual Machine.

Introduction The Java TM 2 Platform Standard Edition (J2SE TM platform) is used for a wide variety of applications from small applets on desktops to web services on large servers.

In the J2SE platform version 1.4.2 there were four garbage collectors from which to choose but without an explicit choice by the user the serial garbage collector was always chosen.